The increasing cost of labor has provided great incentives for expanding the use of automation. However, there are many industrial operations which, because of their complexity, have steadfastly resisted attempts at automatic control. It is those operations that involve the application of a tool to a workpiece and include the requirement of relative movement between the tool and workpiece that are among the most difficult to automate. Examples of such operations are sewing, punching, leather tooling, embroidery and decorative stitching. In all of these operations, it is necessary to move either the tool or the workpiece through intricate patterns. Generally, the tool is in a fixed position and an operator manipulates the workpiece under the tool according to a predetermined pattern. Until recently, automatic movement of the workpiece according to a predetermined pattern has been severly limited.
However, the advent of numerical and direct digital controls has provided readily programmable means for automatically controlling movement of a workpiece through complicated patterns. In such systems apparatus is provided which can support the workpiece for movement along several axes. For example, in an automatic sewing operation, motor operated carriages are provided which respond to pulses from a digital control to support and move the workpiece along two axes. In order to accomplish such movement on a reliably repetitive basis, it was observed that a device for firmly securing the workpiece on the supporting carriage in fixed registration relative to the location of the operating instruments is required. Where the workpieces to be operated upon in the machine are made of relatively hard material such as metal, the workpieces may be readily and securely clamped in a fixed position relative to the operating instruments of the machine. However, where the workpieces are of softer material such as plastic, cloth or leather, the workpieces may be damaged by the securing clamps. Moreover, where the workpiece materials are particularly soft and floppy as with sheets of cloth or leather, accurate holding of the workpiece requires such a plurality of clamps as to make changes between successive workpieces so difficult and time consuming as to vitiate the economic advantage derived from the speed of the automatic controls.
The operator of an automatic stitching machine is no longer required to direct the workpiece under the needle. He need only place the workpiece accurately on the supporting carriage, remove it after the operation is complete, and replace the workpiece. This, however, can be quite difficult when accuracy becomes important as, for example, when a workpiece consists of serveral pieces including overlays and inserts. In the case where overlays are to be sewn to a base the stitching must run close to the border of the overlay, therefore making accurate registration essential.
The stitching machine for which this invention is adapted is comprised of a standard sewing machine from which the feeding means and workpiece support have been removed. These elements are replaced by a numerically controlled motor operated carriage which is capable of movement through a predetermined pattern along two dimensional coordinates which are referenced to the stitching instruments. The pattern is digitized, that is, reduced on a point-by-point basis to known values within the coordinate system and placed on magnetic tape for insertion into the numerical control. The workpiece is mounted on the support carriage in registration with the stitching instruments by means of a pallet.
In this operation, it is anticipated that the operator will have several pallets and proceed as follows: a workpiece will be mounted on the pallet and placed on the supporting carriage for application to the stitching instruments, while the first workpiece is proceeding through the stitching operation a second workpiece will be mounted for processing on an additonal pallet. In this manner substantially continuous operation of the stitching machine will be accomplished.
It should be remembered that in this type of machine the stitching instruments are stationary and the automatically controlled carriage will always be directed in the same path in relation to these instruments. Accordingly, if the operator can place each workpiece in the same position on the carriage, the machine will complete the product without significant variation. This registration problem becomes crucial when overlay or overlapping portions must be secured to the base piece of a composite workpiece as is often required for decorative purposes in the manufacture of shoes. Since such portions need to be stitched as close as possible to their periphery, it becomes essential that the overlay be placed on the pallet in a position which is fixed in relation to the stitching instruments.
It is therefore the object of this invention to construct a pallet which provides means for accurately registering a composite or individual workpiece on an automatically controlled carriage in fixed relation to the stitching instruments. This pallet must also provide means for maintaining this registration during the machine cycle while facilitating removal and replacement of the workpiece without jeopardizing accurate registration.